Alkaline storage batteries can be classified into nickel-cadmium storage batteries, nickel-metal hydride storage batteries, and others, depending on the active material included therein, and into cylindrical, prismatic, and other batteries, depending on the shape thereof. In cylindrical alkaline storage batteries, a spiral electrode group formed by winding a negative electrode and a positive electrode with a separator interposed therebetween is housed together with an electrolyte (liquid alkaline electrolyte), in a cylindrical outer can (battery case) having a side wall including an opening end portion, and a bottom.
One of the current collecting structures of cylindrical alkaline storage batteries is such that: the negative electrode is arranged at the outermost periphery of the spiral electrode group, and the negative electrode at the outermost periphery is brought into contact with the inner surface of the side wall of the metal battery case, allowing the battery case to serve as a negative external terminal; and the positive electrode is connected via a positive electrode lead to a metal sealing member (including a sealing plate and an external terminal plate) for sealing the opening of the battery case, allowing the sealing member to serve as a positive external terminal. For preventing leakage of alkaline electrolyte, a resin packing (gasket) is disposed where the peripheral edge portion of the sealing plate is to contact with the inner surface of the opening end portion of the battery case.
In alkaline batteries (including alkaline storage batteries and alkaline dry batteries) which use an alkaline electrolyte as the electrolyte, however, it is sometimes difficult to seal the opening of the battery case with a resin gasket only without causing leakage in hot and humid conditions. This is due to a possible presence of water at the sealing portion between the opening end portion of the battery case which is electrically negatively charged, and the peripheral edge portion of the sealing plate. The water, if any, causes a reaction represented by the formula “H2O+e−→OH−+½H2” to proceed at the sealing portion. In association therewith, the electrolyte creep phenomenon becomes likely to occur because of the cation migration of metal salts in the alkaline electrolyte, such as K+, Na+ and Li+. Therefore, conventionally, in alkaline batteries, for further improving the sealing by the gasket, a sealant has been provided between the gasket and the inner surface of the opening end portion of the battery case (see Patent Literatures 1 and 2).
Moreover, in alkaline storage batteries being secondary batteries, as compared with alkaline dry batteries being primary batteries, the internal pressure rises as the charge proceeds, and therefore, it is further difficult to prevent electrolyte leakage at the sealing portion.
Patent Literature 1 discloses, as a material of the aforementioned sealant, a soft polymer to which an imidazole compound is added. For the purpose of providing an alkaline dry battery having high impact resistance so that no leakage will occur even when the battery sealing portion is deformed by an impact such as battery drop, Patent Literature 2 suggests using a sealant mainly composed of a polyimide resin and having predetermined adhesiveness or spreadability.
Patent Literature 3 discloses, for the purpose of achieving a higher capacity and a lighter weight by reducing the wall thickness of the container in alkaline storage batteries, the following configuration. Specifically, an upper portion of a cylindrical drum of a bottom-closed metal container of an alkaline storage battery is bent in two steps so as to be flared upward, and an electrode group is housed in the container having a flared cylindrical raised portion. After a liquid electrolyte is housed in the container, an insulating gasket is placed on a step portion under the raised portion of the container. Into the gasket, a sealing plate provided with a terminal cap on its top is placed. Subsequently, the raised portion of the container is reduced in diameter, and then the upper edge of the raised portion is curled. In that way, the sealing plate is compressed and secured via the gasket. Here, the bottom-closed cylindrical metal container satisfies the relationship t1≧t2≧t3 or t1>t2=t3, where t1 is a thickness at a first bend in the direction from the drum portion toward the raised portion, t2 is a thickness of a portion between the first bend and a second bend in that direction, and t3 is a thickness of the drum portion and the raised portion.